My post was overly simplistic, but luckily for me, so is my situation, in that the area I live in is naturally very low humidity. However, we have a method of using this low humidity to cool the house called a "swamp cooler" or evaporative cooler. Basically a big box with fiber pads that has water pumped over them, and a big squirrel cage fan sucks outside air into the house through these soaked pads, and drops the air temp by at least 25 degrees for pennies on the dollar versus A/C.
However, this does raise the humidity somewhat, and anything in the direct air path from the vents in the house gets a heavier dose of moisture. So for like 5 months out of the year, it is a little worse. I have some old (but nice) shotguns on a wall rack in the living room, and those need much more attention than stuff in the safe.
I think the key is staying on top of it, whatever type of preservative you choose, they are all somewhat comparable.
mto's point about dust is important, too - corrosion can be enhanced by electrolytic action of the moisture and dust can increase that, and it keeps the moisture on the metal longer on a small scale.
Jeff